Recently, the need in the exercise treadmill industry for a runaway prevention mechanism was perceived. Runaway may be caused by a motor drive failure. When a failure of that nature occurs, the motor control circuit may output full voltage, thereby causing the motor and, hence, the treadmill to run at full speed with no speed control whatsoever. This was perceived as a dangerous situation. Many treadmill manufacturers subsequently added a runaway prevention circuit to their motor drives.
This runaway prevention circuit was usually a run-of-the-mill "crowbar" circuit of the type found in circuit application books published by various semiconductor manufacturers. A typical "crowbar" circuit as applied to an exercise treadmill is shown in U.S. Pat. No. 5,571,062, issued on Nov. 5, 1996 to Abraham Saganovsky. This patent to Saganovsky shows a silicon-controlled rectifier (SCR) connected across the power terminals of the motor and the output of the speed control circuit for shorting out the motor and the output of the speed control circuit when an abrupt increase in voltage is detected at the output of the speed control circuit.
A problem with the "crowbar" circuit, however, is that it is destructive. When the circuit detects an excessive voltage at the speed control output, it creates a direct short (i.e., "crowbar") across the motor terminals and the output terminals of the speed control circuit. This constitutes a dead short across the line and blows a protective fuse which is normally used. In the process, the "crowbar" circuit usually destroys the rest of the motor speed control circuit.
Additionally, shorting the motor terminals causes an uncontrolled dynamic braking of the motor. This will sometimes damage the motor by causing such things as irreversible demagnetization, lamination melting, spun bearings and the like. Even worse, it could cause the belt of the treadmill to instantly stop, throwing the person on the treadmill forward and injuring him in the process.
It is also the case that the motor might run at full speed for reasons other than failure of the motor speed control circuit. So, if a condition occurs in which the motor speed control is told to go to full output, the destructive runaway prevention circuit (i.e., "crowbar") is activated. This successfully prevents the apparent runaway, but simultaneously destroys the drive circuit and maybe the motor.